Car batteries are basically chemical energy storage units. They are cranked up when the car needs it to run electrical accessories.

There used to be a time when you could simply check how much fluid there was in the battery, but modern barriers do not make this obvious. Even so – if your car won’t start due to a flat battery – there are still some ways to check if your battery requires replacing or if you just need a good old jump start.

Basically, when you are checking if your battery needs to be completely replaced, you’re trying to determine if the battery can properly hold charge any longer. If it cannot do so, then it needs replacing. If it can still hold charge, then you simply need to recharge the battery.

Why do batteries seem to just ‘fail’?

Modern batteries are increasingly compact and more powerful, which is due to an increase in the number of plates contained in each and the subsequent decrease of space between each. This limited space means that it is more likely that a group of plates will "short out" or fail at once, resulting in total failure rather than a gradual decline in performance.

How long has the battery been flat?

If you leave your battery in a discharged condition for longer than a few days, the plates may become "sulphated". This means that sulphate builds up on the plates, inhibiting the ability to hold charge and thus supply the voltage required for the car. Once enough sulphate has built up, the battery will no longer hold charge and becomes useless.

Check the age

While this is not completely guaranteed, it is a rule of thumb good indication. If your battery is more than 4 years old and you are having trouble with it, it most likely needs replacing. Most modern car batteries have an expected lifetime of 4-5 years.

Check for corrosion

The simplest thing to check, if you find your battery is not supplying power, is to look for evidence of corrosion. Take a look at the battery posts and cable ends. Give them a clean and check if this improves the power you get out of the battery. It could be that the energy simply cannot get out of the battery, not that the battery itself is dead.

Check the alternator belts

Next, you’ll want to check the belts that run the alternator. These belts need to be tight enough for the alternator to turn sufficiently in order to charge the battery when the car is running. If the battery is not being properly charged it will eventually go flat, even it the battery itself is in good working order. This may require some technical knowledge, which you may have to seek from a professional or a car enthusiast.

Check the hydrometer

Many car batteries come with a built-in hydrometer. This indicates the level charge in the cell of the battery. A green dot will appear to say that the charge is at sufficient levels. If there is no dot or a dark indicator, the battery is low, but only needs to be recharged. If the dot is clear or yellow then the level of electrolyte is so low that the battery is no longer functional and needs to be completely replaced.

Without a hydrometer, check the voltage manually

You may not have a built-in charge indicator. In this case, you will need to use a digital voltmeter or multimeter to measure the battery’s voltage. Check the voltage after a long drive – it should read above 12.5 volts to be fully charged. Check it again 8 hours later. If the voltage has dropped significantly, then your battery is not able to hold charge and must be replaced. If the drop is not significant, try recharging the battery before replacing it.

Ultimately, if you are unsure of what you are doing, it is best to take your car to a mechanic or auto shop to get the whole system looked at by a professional. They can determine exactly what might be causing your failed battery.